1) Field of the Invention
The invention relates to friction stir welding, and more specifically, to a friction stir welding tool cleaning method and apparatus.
2) Description of Related Art
Friction stir welding is a process that combines extruding and forging and is specifically designed for welding aluminum extrusions and other metal extrusions. Friction stir welding is a process in which a rotating cylindrical tool, that includes a threaded pin or probe attached to a shoulder that may be scrolled, concave or of another suitable configuration, is slowly plunged into or through a workpiece in order to join multiple structural members of the workpiece in a solid state or to repair cracks in the workpiece. FIG. 1 shows a conventional friction stir welding device. As shown in FIG. 1, the tool 10 includes the pin 12 that typically extends from the shoulder 14, which can be concave, flat or otherwise contoured. The shoulder 14 is urged against structural member 16, so that the pin 12 is urged into structural members 16, 18. The pin 12 is then urged through structural members 16, 18 to form a continuous weld joint 20. For example, according to one conventional friction stir welding process for forming a weld joint 20, a friction stir welding machine (not shown) urges the pin 12 in a direction 22 and into an interface 24 defined by two overlapping structural members 16, 18. The rotating tool 10 is then moved in a direction 26 along the interface 24. The motion of the rotating tool 10 generates frictional and deformation heating, thereby forming a region of plasticized material in the structural members 16, 18. This heat causes the work piece material to soften without reaching the melting point and allows the tool to traverse the weld line. As it does, the plasticized material is transferred from the front edge of the welding tool to the trail edge of the tool shoulder and pin, where it joins the pieces being welded. Upon cooling of the plasticized material, the members 16, 18 of the workpiece are joined along the weld joint 20. Because the frictional heat generated by the welding tool plasticizes the material within the weld joint, the plasticized material must be constrained to prevent the material from flowing out of the weld joint and also must be consolidated to minimize porosity and provide a weld joint having the desired surface finish.
Friction stir welding processes typically use either a fixed pin tool or a retractable pin tool. A fixed pin tool is a unitary piece of the pin portion and the shoulder portion. A retractable pin tool is a pin portion that is separate from the shoulder portion, and the pin is inserted into the shoulder and may be adjustable in height. During conventional friction stir welding processes, the fixed pin tool or retractable pin tool collects weld process residue material depending on the material or materials being welded, such as aluminum residue or titanium residue. When using a retractable pin tool during welding, excess weld process residue material builds up between the shoulder's inside diameter and the pin tool's outside diameter and can inhibit the load sensed by a system controller, such as a force control device, possibly causing an over plunge of the pin tool due to false readings in the axial direction. False readings can result in uncertainty about the quality of the weld. In addition, if more load is applied due to a false reading, the tools can over penetrate the surface and dig into the welding components, thus destroying the part. This loading effect can also inhibit correct torque readings making adaptive control much more difficult. When using a fixed pin tool during welding, weld process residue build-up limits the optical inspection of tools and may inhibit a decision to continue or to limit use of that tool. If tools are checked electronically or with a laser system, the residue build-up may imply that the tool integrity is unacceptable when, in fact, a tool change may not be required and the necessary action would be to clean the tool before the inspection process. Friction stir welding tools are easier to inspect for damage if they are cleaned properly.
Known methods for cleaning friction stir welding tools having weld process residue material exist. One known method involves cleaning the friction stir welding tools having weld process residue material with a pick or small screw driver. However, this method is time consuming and tedious, and it may take hours to thoroughly clean the tools. In addition, this method may not remove all of the residue material on the tools. A second known method involves chemically removing the residue through an acid bath process. This cleaning method involves chemically etching the residue material off the friction stir weld tools. For example, with aluminum residue, a sodium hydroxide bath is typically applied. However, this method is also time consuming and can take up to 24 hours to complete. In addition, acid bath residue, such as sodium hydroxide residue, may be present on the outside of the tools after they are chemically etched, and the tools must be additionally heat treated to remove the acid bath residue. A third known cleaning method involves heating the residue build-up with a blow torch and then picking off the heated residue material with a pick or small screw driver. However, this method can be risky and damage to the tool can occur because the high temperature cannot be adequately controlled. For example, if the tools are heat treated at too high a temperature, the heat may anneal the material and alter the heat treatment property of the tools, and then the tools may be damaged and cannot be used in the friction stir welding process. Accordingly, there is a need for an improved friction stir welding tool cleaning method and apparatus that does not have the problems associated with known cleaning methods and devices.